Avalonia/src/Avalonia.Themes.Fluent/Controls/Window.xaml

<Style xmlns="https://github.com/avaloniaui" Selector="Window">
  <Setter Property="Background" Value="{DynamicResource SystemControlBackgroundAltHighBrush}"/>  
  <Setter Property="TransparencyBackgroundFallback" Value="{DynamicResource SystemControlBackgroundAltHighBrush}" />
  <Setter Property="Foreground" Value="{DynamicResource SystemControlForegroundBaseHighBrush}"/>
  <Setter Property="FontSize" Value="{DynamicResource ContentControlFontSize}"/>  
  <Setter Property="FontFamily" Value="{DynamicResource ContentControlThemeFontFamily}" />
  <Setter Property="Template">
    <ControlTemplate>
      <Panel>
        <Border Name="PART_TransparencyFallback" IsHitTestVisible="False" />
        <Border Background="{TemplateBinding Background}" IsHitTestVisible="False" />
        <Panel Background="Transparent" Margin="{TemplateBinding WindowDecorationMargin}" />
        <VisualLayerManager>
          <VisualLayerManager.ChromeOverlayLayer>
            <TitleBar />
          </VisualLayerManager.ChromeOverlayLayer>
          <ContentPresenter Name="PART_ContentPresenter"
                            ContentTemplate="{TemplateBinding ContentTemplate}"
                            Content="{TemplateBinding Content}"
                            Margin="{TemplateBinding Padding}"
                            HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                            VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
        </VisualLayerManager>
      </Panel>
    </ControlTemplate>
  </Setter>
</Style>